The discovery is good news for childhood health as the majority of young people do not meet physical activity guidelines and activity levels tend to decline through childhood.

The researchers, who are studying children’s physical activity in the transitional years between primary and secondary school, also found an extra friend was associated with almost four additional minutes of moderate to vigorous physical activity (MVPA) after school. But the link was only noticed in girls, not boys, suggesting stronger friendship associations for girls than boys.

Dr Russell Jago from the University’s Centre for Exercise, Nutrition and Health Sciences, in the School for Policy Studies, said: “The research shows that children are receptive to being encouraged to undertake more physical activity. We want to encourage young people to be active with their friends and support each other. This information can be used to design strategies to improve activity levels among children at a crucial stage in their development.

“The higher levels of physical activity associated with girls having more friends and having friends who support physical activity suggests promoting activity with friends could be helpful.”

The findings have been published in the journal Medicine & Science in Sports & Exercise by the American College of Sports Medicine. The report noted: “Analysis indicated that an increase in the number of friends between primary and secondary school was associated with increased after-school and weekend MVPA.”

And it concluded: “Increased number of friends and friend support for physical activity was associated with increases in girls’ MVPA after the move to secondary school. Strategies to foster friend support for physical activity may be important for helping girls be active.”

Physical activity was measured with accelerometers worn over seven days and friends were assessed through questionnaires.

While at primary school, boys engaged in approximately 26min of MVPA after school with girls engaging in 21min – a significant proportion of children’s recommended 60min of physical activity a day. At the weekend, boys and girls engaged in 45min and 36min of MVPA respectively.

There was a 16 per cent decline in boys’ after-school MVPA in their first year of secondary school, compared with the primary school values. Girls’ after-school MVPA fell by around 12 per cent. Weekend MVPA for both sexes increased, possibly due to decreased adult outdoors supervision. The reasons for the decline in after-school activity are less clear.

Dr Rachel Thompson, Deputy Head of Science at WCRF, said: “Physical activity is an important factor in reducing cancer risk in later life and it is vital that being physically active is a habit we develop early.

“This study shows physical activity at below recommended levels for primary school children, with a further drop in after-school activity after the move to secondary school which is particularly concerning. It suggests this transition period is critical for improving physical activity levels among children and understanding factors such as the influence of friends is essential.”